We are studying a phenomenon known as hybrid dysgenesis, described by several investigators as a set of aberrant traits including sterility, mutability, and abnormal recombination found in certain hybrids of Drosophila melanogaster. In previous work we have described the associated types of genetic events, the developmental biology of the hybrid sterility, the inheritance of the system which turns out to involve both chromosomal and cytoplasmic components, and an associated case of hypermutability. Further research will be directed toward determining the mechanism behind hybrid dysgenesis and learning the effect of hybrid dysgenesis on the evolution of natural populations with particular emphasis on its possible role in speciation. We have developed a rapid sterility-testing technique which will allow us to obtain a detailed picture of the chromosomal and extrachromosomal heredity involved. It should be possible with these data to eliminate at least some of the hypotheses suggested for the nature of hybrid dysgenesis. To study its effect on natural populations, we ask the following questions: What are the distributions of the chromosomal and cytoplasmic components in the populations? Are these components stable under natural and laboratory conditions? Are the genetic and environmental conditions needed for hybrid dysgenesis found in natural populations? Hybrid dysgenesis appears to be a vastly complex problem important to the understanding of all levels of the biology of Drosophila and perhaps other species.